Introduction

Since investment in brain mass is considered metabolically expensive, its expansion is assumed to be expressed through natural selection only where distinct survival advantages obtain. The mammalian order Cetacea (whales, dolphins, porpoises) is particularly large-brained, encompassing some 68 species of odontocetes (toothed whales) and 11 species of mysticetes (baleen whales). In this study, we examined brain and body size relations in a large number of cetaceans and considered evidence for differences between cetacean species. We also considered variations in size of the cerebellum relative to the whole brain and considered its development relative to body size.

Methods

Whole adult brains were collected and weighed in 18 odontocete and one mysticete cetacean species. Totals of 158 and five individual odontocetes and mysticetes, respectively, were examined. Weights were determined either in the fresh state or following various fixation intervals in 10% buffered formalin. In 12 odontocete and one mysticete species, cerebellar weights were also measured following detachment of the cerebellum at the cerebellar peduncles. All animals were determined to be adults on the basis of mature reproductive tracts and/or published accounts of mature body lengths.

We also used published information on mature body lengths as criteria for adulthood for brain and body mass data from a variety of other cetaceans from the published literature and from several colleagues who had made similar measurements and generously shared their data with us (see acknowledgements). When combined with our own data, our study sample encompassed more than 570 individual adult cetacean brains, representing both extant suborders (mysticetes and odontocetes), 12 families, 32 genera and 46 species. For measurement of cerebellar mass we followed the method of Putnam (1927)¹. The cerebellar peduncles were severed just above the emergence of the seventh and eighth nerves tangential to the brain stem, care being taken to leave the posterior quadrigemina and the fourth nerve intact. Linear regressions were done on various suborder, family, and species groupings to evaluate relationships.

Results

Adult cetaceans vary markedly in body mass. Some individuals of one of the smallest species, the La Plata dolphin, may reach adulthood with a body mass of only 24 kg while some individuals of the largest species, the blue whale, may reach 200,000 kg - a body mass ratio for the largest to the smallest adult cetacean of more than 8000:1.

Adult brain mass also varies between cetacean families from as little as 205 g in an adult-size La Plata dolphin (family Pontoporidae) to 9,300 g in the largest killer whale (the largest member of the family Delphinidae) - a ratio of more than 45:1. The largest average adult brain mass reported, 7,894 g, occurs in the largest odontocetes - male sperm whales.

Brain mass does not vary strictly with body mass, however. In contrast to adult male sperm whales, adult male bowhead whales (a mysticete of the right whale family) that were estimated to have a similar lean body mass (total body mass minus blubber mass) had a brain mass average of 2845 g - a sperm/bowhead ratio of 2.77:1. As a percentage of brain mass, cerebellum mass in our data ranged from 18 to 24% in mysticetes, however, the large percentage of cerebellum is apparently due to a lesser forebrain since relative to body mass the mysticete cerebellum was not as large as that of the larger dolphins such as the killer whale. Sperm whales, beaked whales, and platanistoid dolphins have small cerebellums that range from 6 to 12% of brain mass. Regressions of data from a group of the larger Delphinidae ranging in adult body size from about 100 kg to about 5000 kg were compared to a grouping of Platanistidae, Ziphiidae, and Physeteridae (PZP). The slopes of these two regressions were the same; however, the Y-intercepts were radically different with the delphinia intercept being considerably higher.

Figure 1

Conclusions

Female killer whales (Delphinidae), =1/3 as large, have heavier brains than female sperm whales. More data on large male killer whales are needed to determine which species has the largest brain on the planet.

In a group of Delphinidae, total brain mass and cerebellum mass relative to body mass exceed other groups by = two to three times.

Since brain mass is a metabolic expense, what survival advantage does the larger brain obtain for these Delphinidae?

Acknowledgements

For their contributions of study materials, we thank J. Conrad, L. Cornell, L. Dalton, R. Goldston, N. Hall, E. Huber, A. Krarup, R. Kastelein, K. Keller, R. Lewis, M. Magee, J. McBain, G. Miller, D. Odell, E. Oen, T. Reidarson, M. Semanski, F. Townsend, M. Walsh, and S. Wright. We thank M. Reddy, C. Curry, and R. Smith for technical assistance.

Reference

1.  Putnam I.K. 1927. The proportion of cerebellar to total brain weight in mammals. Proceedings, Royal Acad. Amsterdam XXXI:155-168.